The present invention relates to sliding parts, for example, for use as threaded parts which are screwed on each other, such as union nuts and union screws for screw pipe joints, sliding bearings and shafts to be thereby supported, and two plates Which are slidable on each other.
For example, the union nut and union screw for use in screw pipe joints are so adapted that when these components are screwed on each other, the opposed flanks slide on each other. The union nut conventionally used is formed with a silver plating layer over the surface of its threaded portion in order to prevent scoring or seizure that would occur when it is tightened up.
However, it has been found that the conventional union nut encounters the following problems. If the union nut is repeatedly tightened up on the union screw and unfastened therefrom as when the screw pipe joint is repeatedly installed and removed, the frictional resistance between the sliding faces of the nut and the screw increases, consequently necessitating a gradually increasing torque for tightening up. This entails the problem that a specified tightening axial tension becomes unavailable by controlling the axial tension with the tightening torque. Another problem then arises in that the nut requires an increased torque for unfastening, rendering the pipe joint difficult to remove.
Accordingly, we have conducted various experiments and research to ascertain the cause of the above problems and consequently found that the occurrence of these problems is closely associated with the crystal structure of the silver plating layer, especially with the relationship between the intensity of X-rays diffracted by the (111) plane thereof and the intensity of X-rays diffracted by the (200) plane. When the intensity of X-rays diffracted by the (111) plane of the silver plating layer is taken as 100, the intensity of X-rays diffracted by the (200) plane is less than 30 in the case of the conventional union nut. We have found that this results in the foregoing problems.